1. Field of the Invention
The present invention relates to a support frame for a glass sheet heated beyond its deformation temperature or point and in particular a frame for recovering a glass sheet after its convex bending or cambering; and for transferring the glass sheet from the molding cell to the tempering cell, where it undergoes intense cooling on its two faces in order to improve certain of its mechanical characteristics. The invention particularly applies to the production of safety glass windows for cars, such as side windows, opening roofs or rear windows.
In cambering-tempering processes carried out in the horizontal position such as those described in French Patents 2,085,464, 2,596,751 and 2,567,508 or European patent 3,391, the glass sheet is passed through a reheating furnace up to a molding unit, where it is vertically displaced in order to abut against an upper cambering mold, whose contours it essentially adopts. The at least partly shaped glass sheet is then released onto a support frame, which is then called the tempering frame and which transfers the glass sheet to the tempering means.
2. Discussion of the Background Art
In the aforementioned patent publications, the glass sheet is maintained against the upper cambering mold by pneumatic forces generated by an upward hot air stream (FR-A-2 567 508) or by suction on the periphery of the glass plate or window (FR-A-2 085 464 or FR-A-2 596 751), or by suction over the entire glass surface (EP-A-3 391). In order to pass the glass sheet from the upper cambering mold to the tempering mold, the pneumatic force generators are cut off, so that the glass sheet is then only subject to its own weight and gravity makes it fall onto the support frame. However, the instant of the fall, or more specifically, the instant at which the glass sheet loses all contact with the upper cambering mold, does not precisely coincide with that of the stoppage of the pneumatic forces. Thus, the application of the glass sheet to the upper cambering mold generates frictional forces, which tend to hold the glass sheet. However, these frictional forces have amplitudes which vary as a function of the curvature of the upper cambering molds (the frictional forces increasing as the cambering mold approaches a basket handle shape) and also the wear to the interposed paper or fabric sheet conventionally used for covering the upper cambering mold and which constitutes the contact surface with a glass.
The consequence of this phenomenon is not only that the glass sheet drops systematically with a certain delay, but also with a delay which differs between the individual glass plates. Therefore, the time spent by the glass sheet on the support frame varies. If the fall of the glass sheet takes place rapidly, the residence time is longer and the glass will deform on the support frame, which creates an undesired double cambering. However, if the drop is slower or delayed, the fall can take place at an inopportune time, for example, when the frame is already moving towards the tempering cell. The sheet is then badly positioned on the tempering frame, which leads to optical defects and/or to incomplete tempering. In the worst possible case, the glass sheet may not even be recovered by the frame, which causes the problem of the removal of cullet below the molding cell, particularly when using a rising hot air stream for raising the glass plate.
In order to remedy these disadvantages, it is known inter alia from European Patent 3,392 to bring about the dropping of the glass sheet not only by eliminating the forces maintaining it against the upper cambering mold, but also by forcing the same through the application of a gaseous pressure. This is a suitable process for upper cambering molds equipped with a plurality of suction orifices. It can easily be realized in this case by providing suction pumps, which are reversible and by using the suction orifices as blowing orifices. However, for other upper mold types, i.e., for upper molds not equipped with suction orifices, it is much more difficult to put into effect, because it is necessary to provide a complimentary blowing circuit.